Squash mosaic virus (SqMV) is a member of comovirus group, with isometric virus particles about 30 mm in diameter. Virus capsid is composed of two distinct polypeptides with molecular weights of 22 and 42 kilodaltons. A viral genome of members of comovirus group consists of two single-stranded, positive-sense RNA molecules identified as middle-component RNA (M-RNA) and bottom-component RNA (B-RNA) of ca. 4200 and 6000 nucleotides, respectively. Both M-RNA and B-RNA are polyadenylated at the 3'-ends and have a genomic-linked protein (VPg) at the 5'-termini. The RNAs are translated into polyproteins from which the functional proteins are derived by proteolytic cleavages.
SqMV induces diseases on squash and melons, and is transmitted by beetles and through seeds. Control of the virus is normally through the use of virus-free seeds and insecticides for the control of beetles. While these methods can achieve some level of success in reducing the level of infection in a crop, they are time consuming and relatively expensive. However, no SqMV-resistant genes have been identified in squash or melons which would render plants carrying such genes resistant to SqMV infection. It is desirable to have strains of squash and melon plants which are resistant to SqMV infection. According to the present invention, SqMV infection in melons and squash is controlled by coat protein-mediated protection.
Expression of the coat protein genes from tobacco mosaic virus, alfalfa mosaic virus, cucumber mosaic virus, watermelon II mosaic virus, papaya ringspot virus, zucchini yellow mosaic virus and potato virus X in transgenic plants has resulted in plants which are resistant to infection by the respective virus. In order to produce such transgenic plants, the coat protein gene must be isolated and inserted into the genome of the plant. Furthermore, the coat protein gene must contain all the genetic control sequences necessary for the expression of the gene after it has been incorporated into the plant genome.
One distinction between SqMV and the other viruses listed above is that SqMV virus capsid is composed of two different types of coat proteins whereas the other viruses have a single type of coat protein. It is necessary to determine if the expression of one or both coat protein genes is required for coat protein-mediated protection. This necessitates identification of the coat protein genes, engineering the genes into plant expression and transformation vectors, transforming plants to express the coat protein genes singly or doubly, and testing plants for resistance to infection.
In the present invention, the nucleotide sequences of the coat protein genes for SqMV have been determined and the genes have been inserted into expression vectors to supply them with the necessary genetic regulatory sequences so that the genes can be expressed when incorporated into a plant genome. Plant cells are transformed with the vector construct and the plant cells are induced to regenerate. The resulting plants express the coat protein genes that they contain. The transgenic plants according to the present invention are resistant to infection by the SqMV.